Solenoids are commonly-used, electrically actuated transducer devices used to convert electrical energy into linear motion. For example, solenoids can be used as part of a starter in a vehicle, or as part of control systems for other electromechanical devices in which a generally short motion is required. Generally, solenoids include a movable component (such as a plunger) movable between two positions, and biased into one of the two positions (e.g., by a spring). Movement of the plunger to the second position is actuated by current passing through a coil, which induces a magnetic field causing a force to counteract the spring force.
In some applications, solenoids are required that have relatively high pull-in forces and/or long strokes. For example, solenoids used in aviation-type applications, typically are required to have a very strong hold force and are required to have a long actuation stroke. One example of such a high-force, long stroke solenoid is illustrated in FIG. 1. In that arrangement, a solenoid 10 has a solenoid body 12. A plunger 14 is retained partially within the body 12, and movable between extended and pull-in positions. The plunger 14 is biased into the extended position by a spring 16 positioned within the body 12. A coil 18 is also positioned within the body and configured to engage the plunger 14 when it receives a signal (e.g., a current through the coil) of a sufficient magnitude to generate a magnetic signal to counteract the force of the spring 16, thereby moving the plunger 14 to the pull-in position. The solenoid 10 can have a separate interface, shown as the connector 20, by which wires can be connected to the coil 18 for control of the solenoid 10.
These types of solenoids typically have a pull-in force of over 14 lbs. (e.g., 15 lbs.) and strokes (i.e., distance between actuated and non-actuated positions) of over about ¼ inches require special design considerations. The high forces and long strokes drives coil design to a high number of turns, which increases the solenoid weigh, volume, and power dissipation. For example, typically such solenoids require in excess of 2,000 turns in the coil, and may require a hold force of up to 100 lbs.
However, these high forces require high currents to generate a magnetic field of sufficient magnitude; at the same time, a low operating current is required to minimize coil heath, and decrease the electrical power demanded from the system powering the solenoid. In some cases, this demand can be about 45 watts to actuate the solenoid from its extended position to a pull-in position, and 14 watts (continuously) to hold the solenoid in the pull-in, actuated position.
In order to mitigate the high power demands of such specialized solenoids, current is reduced across the coil during a hold period, because the hold force required for a solenoid is generally lower than an actuation force. An example of such a circuit is illustrated in FIG. 2. In this arrangement, the solenoid 10 includes a solenoid coil 18 having a resistance 18a, and which is connected in parallel with a diode 22. A voltage can be applied across the solenoid 10 and diode 22, from a voltage connection to a ground (powered) connection. An additional resistor 24 is selectively connectible in series with the combined solenoid 10 and diode 22, to a second ground (hold) connection. To actuate the solenoid, a high current is distributed across the voltage and ground (powered) connection, thereby maximizing the current across the solenoid coil, and actuating the solenoid. Once the solenoid 10 is actuated, an external controller can switch the circuit used such that the same voltage is applied across the voltage and ground (hold) connection, thereby introducing resistor 24 into the circuit and lowering the overall current through the coil 18.
However, even these existing solenoid designs have various disadvantages, because in these circumstances the solenoid still is required to have a very large number of turns and requires a high hold force, thereby dictating that the solenoid is bulky and energy-inefficient.
For these and other reasons, improvements are desirable.